This invention relates to an aerodynamic exterior rear view mirror for motor vehicles and especially for large vehicles such as trucks and busses that will enable the operator to view trailing traffic and to assist in backing up the vehicle. The exterior mirror for large trucks and busses must be supported from the vehicle at a location spaced away from the vehicle to enable the operator to see around the trailing end of the vehicle. Thus the exterior mirror is located in the vehicle created flow field and creates drag that must be overcome by thrust created by the vehicle's engine. The mirror of this invention has an aerodynamically shaped exterior mirror head that carries an adjustable mirror therein. The reflecting surface of the mirror is located within the mirror head creating a flat rear surface for the mirror head. As a result, the smooth aerodynamic surface of the mirror head comes to an abrupt halt, along trailing vertically extending edges. The abrupt end to these surfaces will result, in an idealized flow situation (frictionless flow), in the airflow continuing to the trailing edges with separation occurring at the trailing edges followed by a turbulent wake. The wake is the primary source of mirror head drag (surface friction being a small secondary drag source) and its size determines the magnitude of drag for a given free stream velocity. The size of the wake is determined by the location of the point of separation. In the idealized case, the point of separation is at the trailing edge and the size of the wake is dependent upon the attitude of the surfaces the airflow is following relative to the direction of the local free stream airflow. The term "local free stream airflow" refers to the movement of air relative to the side of the vehicle. In the actual, physical world the point of separation may be at the trailing edges of the mirror head or at some point upstream of the trailing edges and is a function of both the attitude of the surfaces that the airflow is following relative to the direction of the local free stream airflow and the characteristicsof a thin, naturally occurring, surface boundary layer. The flow field around the aerodynamic mirror head is basically laminar by nature, however, within this flow field, on the external surface of the mirror head, a thin layer of slower moving air, called a boundary layer, results from the friction created between moving air and a non-moving surface, creating friction drag which is small relative to the wake drag. This boundary layer gradually thickens as the air within flows downstream towards the mirror head trailing edges. The flow within this boundary layer is initially laminar and remains so until some surface irregularity causes a transition to a turbulent boundary layer or an adverse pressure gradient causes a localized or complete separation. The surface location of this point of separation determines the size of the downstream wake. When the inboard and outboard surfaces of the mirror head are diverging immediately prior to the trailing edges, relative to the local free stream airflow, a relatively large turbulent wake related to the divergence angle, will be created beyond the trailing edges. The boundary layer in this case will probably remain attached and separation will occur at the trailing edges. If the surfaces are parallel immediately prior to their trailing edges, relative to the direction of the local free stream airflow, and the airflow remains attached, the wake size will be reduced in size as compared to that of the diverging trailing edges with a corresponding reduction in drag. If the boundary layer separates from the surface forward (upstream) of the trailing edges, the size of the wake will increase and potentially reach or exceed the wake size of the diverging surface wake. If the surfaces are converging immediately prior to their trailing edges, relative to the direction of the local free stream airflow, and the airflow remains attached, the wake size will be substantially reduced to a wake closing size much smaller than that of the parallel trailing edges size with a correspondingly larger reduction in drag. In this situation, however, upstream boundary layer airflow separation is very likely. In all three cases, without boundary layer flow separation, the surface convergence immediately prior to their trailing edges will result in the least amount of mirror head aerodynamicdrag. In all three cases with boundary layer flow separation, the mirror head drag will be higher. The further forward the separation point, the larger the wake and the higher the drag. These high drag forces increase as the vehicle speed increases resulting in greatly increasing fuel consumption.
The U.S. Pat. No. 5,179,470 discloses the concept of providing shaped vanes that are located adjacent the trailing edges of the mirror head that function to reduce the downstream wake. However, the invention of this patent controls the size of the wake of the main mirror head body by channeling the boundary layer within the turning vane flow field thereby reducing the size of the wake to the area size of the trailing vertical edges or possibly less. However, the shape design of the vanes is very critical less they generate their own separated flow field.
The U.S. Pat. No. 5,069,538 discloses a side mounted rear view mirror for a motor vehicle that includes a small air deflector disposes upstream of the mirror housing for the purpose of creating an air stream that is smoothly channeled past the lateral edges of the mirror housing without obstructing the operators view. This rear view mirror does not prevent the problem of high draw beginning from the point where separation of laminar flow occurs to the vertically extending edges or the following turbulent wake.
For the foregoing reasons, there is a need for an aerodynamic exterior rear view mirror for motor vehicles in which separation of the airflow is eliminated or the separation point contained to a minimum distance from the trailing edges and the size of turbulent wake beyond the trailing edges is minimized resulting in a substantial drag reduction.